The present invention relates to arc flash prevention and mitigation technologies, and particularly relates to electrical crowbar devices for this purpose.
Electric power circuits and switchgear have conductors separated by insulation. Air space often serves as part or all of this insulation in some areas. If the conductors are too close to each other or voltage exceeds the insulation properties, an arc can occur between conductors. Air or any insulation (gas or solid dielectrics) between conductors can become ionized, making it conductive, which enables arcing. Arc temperature can reach as high as 20,000° C., vaporizing conductors and adjacent materials, and releasing an explosive energy that destroys circuits and endangers personnel, even at a distance. For example, an arc of 10 kA/480V has the energy of about 8 dynamite sticks. Approximately five to ten serious arc flash incidents occur in the United States daily.
Arc flash is the result of a rapid energy release due to an arcing fault between phase-phase, phase-neutral, or phase-ground. An arc flash can produce high heat, intense light, pressure waves, and sound/shock waves similar to that of an explosion. However, the arc fault current is usually much less than a short circuit current, and hence delayed or no tripping of circuit breakers is expected unless the breakers are selected to handle an arc fault condition. Agencies and standards such as the National Environmental Policy Act (NEPA), Occupational Safety and Health Administration (OSHA), and Institute of Electrical and Electronics Engineers (IEEE) regulate arc flash issues through personal protective clothing and equipment, but there is no device established by regulation to eliminate arc flash.
Present arc flash mitigation technologies are not satisfactory. Standard fuses and circuit breakers do not react fast enough to an arc flash. An electrical “crowbar” is a protection device that intentionally shorts an electrical circuit and thus diverts the electrical energy away from the arc flash. The intentional 3-phase short circuit fault thus created is then cleared by tripping a fuse or circuit breaker, finally shutting down the power. Examples of prior art arc flash mitigation devices are disclosed in U.S. Pat. Nos. 6,141,192, 6,532,140, 6,633,009, 6,839,209, and international patent publication WO9921254. However, circuit breakers that rely on mechanical and/or electro-mechanical processes may be too slow to stop an arc flash before damage occurs.